1. Field of the Invention
The present invention relates to a thin film field-effect transistor and a display using the same. Specifically, the invention relates to a thin film field-effect transistor using an amorphous oxide semiconductor in an active layer and a display using the same.
2. Related Art
In recent years, due to progress in liquid crystal and electroluminescence (EL) technologies and the like, flat panel displays (FPD) have been put to practical use. In particular, since high intensity light emission can be obtained by low voltage with an organic electroluminescent device (hereinafter sometimes also referred to as “an organic EL device”) using a thin film material that is excited and emits light by applying an electric current, effects such as reduction in device thickness, weight and size, energy saving and the like are expected in broad fields including portable telephone displays, personal digital assistants (PDA), computer displays, information displays of automobiles, TV monitors, and general illumination.
These FPDs are driven by an active matrix circuit of a field-effect thin film transistor (hereinafter sometimes referred to as a thin film transistor or TFT) using an amorphous silicon thin film or a polycrystalline silicon thin film in an active layer provided on a glass substrate.
On the other hand, attempts have also been made to use a light weight and flexible resin substrate in place of a glass substrate in order to achieve further reduction in thickness, reduction in weight, and improvement in breaking resistance of these FPDs.
However, manufacture of the above transistors using silicon thin films requires a thermal process of relatively high temperature, and it is generally difficult to directly form a thin film on a resin substrate that is low in heat resistance.
Accordingly, a TFT using an amorphous oxide capable of film-forming at a low temperature, e.g., In—Ga—Zn—O series amorphous oxides, as a semiconductor thin film has been actively developed (e.g., refer to Japanese Patent Application Laid-Open (JP-A) No. 2006-165529), and IDW/AD '05, pp. 845-846 (Dec. 6, 2005)).
A TFT using an amorphous oxide semiconductor is capable of film-forming at room temperature and can be formed on a film, and therefore it has recently been attracting attention as the material of an active layer of a film (flexible) TFT. In particular, it has been reported by Hosono et al. of the Tokyo Institute of Technology that a TFT using a-IGZO has a field-effect mobility of about 10 cm2/Vs even on a PEN substrate, which is higher than that of an a-Si series TFT on a glass substrate (e.g., refer to NATURE, Vol. 432, pp. 488-492 (Nov. 25, 2004)), and such a TFT has been receiving attention in particular as a film TFT.
However, an amorphous oxide semiconductor layer is liable to cause oxygen defects, and has problems such that the layer fluctuates according to the atmosphere at the time of film forming, semiconductor characteristics are variable due to plasma irradiation and the like at the time of film forming, or performances fluctuate during storage after manufacture. Further, there is a problem such that an amorphous oxide semiconductor layer is liable to be corroded by an acidic etchant for patterning, and a patterning process by photolithography cannot be used, so that highly refined patterning is difficult.
As a method for solving these problems, it is reported that oxygen defects can be suppressed by, for example, providing an amorphous oxide insulating layer as a protective layer, exactly controlling the oxygen concentration in a deposition process, and controlling to include 3.8×1019 pieces/cm3 or more of desorbed gas observed as oxygen by thermal desorption spectroscopy (e.g., refer to JP-A No. 2008-166716). It is also reported that oxygen defects can be suppressed by including a first process of forming a semiconductor layer in an atmosphere having an introduced oxygen partial pressure of 1×10−3 Pa or less, and a second process of performing heat treatment (at 150° C. to 450° C.) in an oxidizing atmosphere after the first process to restore the oxygen defects that have occurred in the semiconductor layer-forming process by the second process (e.g., refer to JP-A No. 2008-53356). However, since both methods require exact control of process conditions in the control of oxygen defects, the processes are complicated, the apparatus are large-sized, and strong heat treatment cannot be applied to a TFT using a film (flexible) substrate. Further, the problems of etchant-resistance in a patterning process and storage stability of a semiconductor film after manufacture remain unresolved.